The invention consists of copolymers with their maximum absorption wavelength below 350 nm, usable in non-linear optical applications. More specifically, it concerns new families of amorphous polymers whose lateral chains contain chromophoric groups which, when oriented by a steady electrical field at a temperature close to the vitreous transition temperature, give rise to second-order non-linear optical effects. These effects can be used to produce integrated optical components such as a frequency doubler operating on wavelengths between 0.8 and 2 .mu.m or an electrooptical modulator controlling an electromagnetic wave whose wavelength can be between 0.6 and 2 .mu.m.
At present, it seems that "fully optical" solutions will, to a large extent, be used for future telecommunication systems and new generations of data processing systems. Passive, optically active, optoelectronic and optooptical components must be developed to match this trend.
A present, most electrooptic systems are manufactured using lithium niobate or semi-conductor materials.
However, it has now been clearly proved that many organic materials possess a greater non-linear optical activity than inorganic materials. The polarization P induced by intense illumination can be written: EQU P=.parallel..sub..chi. (1).parallel.E+.parallel..sub..chi. (2).parallel..vertline.E.vertline.E+. . .
where
.sub..chi. (2) is a second-order susceptibility tensor. PA1 E is the electromagnetic field applied to the medium. PA1 monocrystals PA1 Langmuir-Blodgett films PA1 polymers. PA1 a polymer chemical structure whose maximum absorption wavelength is below 350 nm PA1 d.sub.33 &gt;5 pm/V (a factor representing the second-order susceptibility) PA1 a vitreous transition temperature above 80.degree. C. PA1 films produced from polymers with good mechanical strength and optical properties (no scattering and good transparency to the wavelength used).
Moreover, due to the fact that the non-linear effects in organic materials are purely of electronic origin, the response times obtained with these materials are very short. Moreover, optical damage only occurs at far higher levels than in inorganic materials. Finally, the chemical structure of the organic compounds can be easily varied to adjust, in particular, the transparency and the .beta. hyperpolarizability value of these molecules; these two parameters vary in opposite directions.
Non-centrosymmetric materials obtained using organic molecules which produce second-order non-linear effects are:
Of these three possibilities, polymers seen the most promising since they are relatively easy to use and, consequently, inexpensive. They can be applied to large surfaces and are compatible with inorganic substrates such as semi-conductors. This combination of properties suggest they are the best material for integrated optics.
The first polymer materials proposed were solid solutions of molecules with non-zero .beta. hyperpolarizability in a amorphous polymer matrix (K. G. Singer, J. F. Sohn and S. J. Lahama, Appl. Phys. Lett. 49,248 (1986)), H. L. Hampsch, J. Yang, G. K. Wong and J. M. Torkelson, Macromolecules, 21,526 (1988)) or a liquid crystal polymer matrix with lateral chains. To increase the non-linear entity content, new polymers have been produced (the applicant's patent FR 2 597 109).
Up to present, considerable efforts have been devoted to obtaining materials with a high .sub..chi..sup.(2) susceptibility in polymers offering second-order non-linear effects. This implies that the chemical structures in the entity which generate the second harmonic must have a high .beta. hyperpolarizability value and thus decreases the transparency of the material in the visible range. Consequently, these materials can only be used at wavelengths above 1 .mu.m in frequency doublers and 0.6 .mu.m in electrooptical modulators.
In the prior art, homopolymers with the chemical structure shown in FIG. 1, whose maximum absorption is below 350 nm, have been synthesized (Eur. Polym. J. 18 651 (1982). These materials have sufficient second-order susceptibility to be used for some applications. However, they have low vitreous transition temperatures, which tends to cause relaxation of the lateral chains after orientation and gives rise to smectic or nematic mesophases which cause light scattering phenomena unacceptable in guided-optic applications.
To satisfy the following criteria: